In the field of high-speed printing devices of the type which are especially suitable for use in connection with electronic business systems, the wire matrix type of printer has come into increasing use. In this type of printer, letters, numbers and symbols are formed from a series of dots produced by the impact of the ends of a plurality of wire elements on record media.
A wire matrix printer of the type that may be used with the present invention is disclosed in U.S. Pat. No. 3,882,985 entitled "Tiltable Matrix Print Head to Permit Viewing of Characters", by George N. Liles. Each of the individual wire printing elements of the wire matrix printer is driven by a solenoid that is energized when a printing stroke of that particular wire is required. A solenoid is generally defined as an electrically energized inductor which may consist of one or more layers of windings so as to form an electromagnet. A circuit for driving the solenoids must therefore be capable of quickly driving the solenoid with an adequate and repeatable magnitude of force and in addition the solenoid driving circuit must enable a rapid recovery of the solenoid from the printing stroke in preparation for the next stroke. This must all be done while simultaneously protecting the solenoid winding against damage from overheating. In such applications we are therefore faced with conflicting requirements, namely a high voltage must be applied to the solenoid in order to decrease its activation time, but this high voltage, in turn, will cause a high level of current to flow through the solenoid winding causing heating in excess of design limits.
In U.S. patent application Ser. No. 627,736 entitled "Drive Circuit", by John W. Stewart et al., which application is assigned to NCR Corporation, the assignee of the present invention, there is disclosed a circuit for coupling a supply voltage to a solenoid so as to inhibit the circuit if the supply voltage exceeds predetermined variations.
Another prior art circuit of interest is disclosed in U.S. Pat. No. 3,549,955 entitled "Drive Circuit For Minimizing Power Consumption In Inductive Load" by T. O. Paine. The circuit disclosed in the patent utilizes two differing threshold voltage levels, one of which initially allows the driving voltage applied to the solenoid to be applied for a period which is sufficient to permit the current through the solenoid to exceed the "pull-in" current. The circuit then automatically terminates the driving voltage and the current through the solenoid is permitted to decay to a value just exceeding the "drop-out" current. The circuit then continues to cycle on and off to alternately drive current through the solenoid and to permit it to decay.